The invention relates to a drive mechanism for mowing knives moving to and fro on harvesting machines.
There is a wide variety of designs of knife drives of this type. The simplest is a crank which converts a rotary movement into an oscillating movement via a crank arm or connecting rod. For reasons of space, this type cannot be used on self-propelled working machines, such as, for example, a combine-harvester.
To make it possible to have a narrower overall size, on these machines the crank mechanism is often deflected through 90.degree. via a rocker.
For other constructions, it is preferable to use swashplate bearings, the wobbling motion of which generates oscillating movements via a swashplate shaft moving to and fro at right angles to the wobble axis, and via levers which are fastened, in turn, to this swashplate shaft. The disadvantage of all these constructions is that the transmission of force to the mowing knife is not exactly linear. Each rocker and each lever of the types of drive described execute a radial movement about their centre of rotation. The longer the particular lever, the smaller the radial movement, but the higher the torque which is exerted on the centre of rotation when the knife is subjected to load. The smaller the radial movement, the longer the lever arm. The longer the lever arm, the higher the torque. The higher the torque, the stronger and heavier the drive elements must be. These in turn are the more costly, the more sturdy they are, and more weight, space and material are required for them. Drives of this type are, from the outset, only suitable for as short a stroke of the mowing knife as possible.
A much more favourable possibility is afforded by a drive mechanism which is similar to a planet gear and in which a planet wheel rolls in a fixed inner toothed ring, this planet wheel being mounted on a rotor, and a crank being coupled to the planet wheel. The radius of the inner toothed ring and the rolling-circle diameter of the planet wheel are of equal size. The radius of the rolling circle of the planet wheel is, in turn, equal to the radius of the crank which is connected operatively to the planet wheel.
As a result of the constructive design, the crank-pin executes an absolutely straight to-and-fro movement during each revolution of the rotor. This linear movement thus corresponds exactly to the diameter of the rolling circle of the inner toothed ring or to double the diameter of the planet wheel or to four times the radius of the crank connected to the planet wheel.
Consequently, the larger the inner toothed ring and planet wheel are made, the greater the stroke to be executed by the mowing knife.
Because of the very wide cutting mechanisms customary at the present time, with correspondingly long and heavy mowing knives, the associated vibrations of the knives moving to and fro restrict the number of strokes. If the length of stroke is increased, together with a greater number of cuts because a particular blade runs over more than one opposing cutting edge, the number of load alternations can be reduced sharply, without a reduction in the mowing capacity. If, for example, the stroke length is double, the number of strokes can be halved, but the cutting capacity nevertheless remains the same.
However, the mechanisms known hitherto, with a fixed inner toothed ring and a planet wheel, have a very wide and high overall size, especially when a long stroke is to be executed. On the one hand, the toothed ring has to be fastened in a housing, thus necessitating a correspondingly large housing, and on the other hand the planet wheel has a double mounting in the rotor, with the result that the rotor bearing has to have a very large diameter because the mounting of the planet-wheel shaft requires a correspondingly large rotor diameter. Bearings of this type are very expensive. In the constructions described, the mounting of the planet wheel and the connecting elements between the planet wheel and the crankpin also involve a very high outlay. Moreover, the assembly and dismantling of this type of mechanism are very complicated and entail a great amount of work.
The patent application No. P 34 27 503.7 already described such a drive mechanism for the mowing knives of harvesting machines, in which the planet wheel is mounted on a bracket attached to the rotor.
The patent application No. P 35 25 576.5 proposed mounting the planet wheel on a bracket by means of an inner bearing and in a bore of the rotor by means of an outer bearing, in order thereby, on the one hand, to make the distance between the two bearings of the planet wheel as long as possible and consequently the stability of this mounting as high as possible and, on the other hand, to keep the bearing of the rotor as small as possible for reasons of cost.